Production process for water-absorbent resin

ABSTRACT

The present invention provides a process for producing a water-absorbent resin of which the residual monomer content and the extractable content are low. The process for producing a water-absorbent resin comprises the step of polymerizing at least one monomer component including acrylic acid and/or its salt as major components to produce a water-absorbent resin that is a neutralized salt, and this process is characterized in that the acrylic acid is a product obtained by catalytic gas phase oxidation of propylene and/or propane and has a protoanemonin content of not more than 10 ppm, and in that the resultant water-absorbent resin has a neutralization ratio of not less than 50 mol %.

This application is a divisional application of Ser. No. 09/883,266,filed Jun. 19, 2001, now U.S. Pat. No. 6,927,268.

BACKGROUND OF THE INVENTION

A. Technical Field

The present invention relates to a process for producing awater-absorbent resin of which the residual monomer content and thewater-extractable content are low.

B. Background Art

Water-absorbent resins are widely utilized as absorbent articles, suchas disposable diapers and sanitary napkins, by being combined withfibrous materials such as cotton, pulp, paper, and sponge.

Industrially most commonly used as the water-absorbent resins areacrylic acid-based crosslinked polymers, such as crosslinked polymers ofpartially neutralized poly(acrylic acids), because of their high waterabsorbency wherein the acrylic acid-based crosslinked polymers areobtained using acrylic acid and/or its salt as monomers (e.g.JP-A-054751/1987, JP-A-031306/1991, JP-A-211934/1994, U.S. Pat. No.4,654,039, U.S. Pat. No. 5,338,810, U.S. Pat. No. 5,574,121, U.S. Pat.No. 5,562,646, EP 0574260 and EP 0942014).

As is mentioned above, the acrylic acid-based polymers are most commonlyused because of their high water absorbency. However, their use isdirected mostly to the above sanitary materials. Therefore naturally inthis field the acrylic acid-based polymers are required to have noproblems. The acrylic acid-based polymers are obtained by a processincluding the step of polymerizing at least one monomer componentincluding acrylic acid and/or its salt as major components, and theresultant polymers contain the unreacted residue of the acrylic acidmonomer although its content is very small. Therefore, the decrease ofthis content is required. As to the water-absorbent resin, it furtherneeds to be water-swellable and water-insoluble, but a very small amountof water-extractable component which is a water-soluble polymer is alsocontained in the water-absorbent resin, and this water-extractablecomponent has a bad influence on water absorption properties, thereforethe decrease of the amount of this water-extractable component is alsorequired. Further demanded to the water-absorbent resin are propertiesunder a load, such as water absorption capacity under a load and liquidpermeation quantity under a load.

SUMMARY OF THE INVENTION

A. Object of the Invention

Thus, an object of the present invention is to provide a process forproducing a water-absorbent resin of which the residual monomer contentand the water-extractable content are low.

B. Disclosure of the Invention

A process according to the present invention for producing awater-absorbent resin, which solves the above problems, comprises thestep of polymerizing at least one monomer component including acrylicacid and/or its salt as major components to produce a water-absorbentresin that is a neutralized salt, and this process is characterized inthat the acrylic acid is a product obtained by catalytic gas phaseoxidation of propylene and/or propane and has a protoanemonin content ofnot more than 10 ppm, and in that the resultant water-absorbent resinhas a neutralization ratio of not less than 50 mol %.

The water-absorbent resin obtained by the above process according to thepresent invention can have a water absorption capacity of not less than25 g/g under a load (of about 1.96 kPa).

Another process according to the present invention for producing awater-absorbent resin comprises the step of polymerizing at least onemonomer component including acrylic acid and/or its salt as majorcomponents to produce a water-absorbent resin that is a neutralizedsalt, and this process is characterized in that the acrylic acid used asa raw material is a product being obtained by catalytic gas phaseoxidation of propylene and/or propane and containing an aldehyde, andfurther characterized by further comprising the step of subjecting theraw acrylic acid to a strong-alkali treatment followed by thepolymerization step.

These and other objects and the advantages of the present invention willbe more fully apparent from the following detailed disclosure.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a section which illustrates a measurement apparatus for theliquid permeation quantity under a load.

DETAILED DESCRIPTION OF THE INVENTION

As is mentioned above, the process according to the present inventionfor producing a water-absorbent resin is characterized by comprising thestep of polymerizing at least one raw monomer component includingacrylic acid and/or its salt as major components wherein theprotoanemonin content (favorably also the furfural content, morefavorably also the total of both contents) of the acrylic acid is notmore than 10 ppm, or by further comprising the step of subjecting theacrylic acid to the alkali treatment followed by the polymerization stepwherein the acrylic acid contains an aldehyde. Therefore, hereinafter, adetailed explanation is first made about a process for producing theabove raw acrylic acid and a process for obtaining the acrylic acidhaving a low furfural and/or protoanemonin content from acrylic acid.

Examples of known industrial processes for producing acrylic acidinclude a process of gas phase oxidation of propylene and/or propane, anethylene cyanohydrin process, a high pressure Reppe process, an improvedReppe process, a ketene process, and an acrylonitrile hydrolysisprocess. Of them, the process of gas phase oxidation of propylene and/orpropane is most commonly employed. Then, the present invention isfavorably used for improvement on the process for producing awater-absorbent resin which involves the use of acrylic acid obtained bysuch a gas phase oxidation process. Incidentally, propane intrinsicallybehaves similarly to propylene, therefore hereinafter the above gasphase oxidation process is explained by citing propylene as arepresentative example.

The process of gas phase oxidation of propylene and/or propane is aprocess in which acrylic acid is obtained from propylene via acrolein bya catalytic gas phase oxidation process. The reaction is usually runthrough two steps using a diluent such as water vapor or inert gas.There are advantages in that the conversion of propylene is almost 100%,so the yield is high. In the propylene gas phase oxidation process, anacrylic acid solution is usually obtained by causing a solvent to absorbthe resultant reaction gas. Examples of the solvent as used in thisprocess include conventional solvents such as water,organic-acid-containing water, and inert hydrophobic organic liquidshaving high boiling points (for example, diphenyl ether, diphenyl). Thisacrylic acid solution contains a large quantity of solvent, which istherefore removed with a solvent-separating column to obtain desolvatedacrylic acid. In this step, if the major component of the solvent iswater, acrylic acid which is substantially free of water is obtained asa distillate from the distillation column by azeotropic distillationusing a solvent which is azeotropic with water. Or if the majorcomponent of the solvent is the inert hydrophobic organic liquid havinga high boiling point, acrylic acid which is substantially free of thissolvent is obtained from a middle or top portion of the distillationcolumn.

The above acrylic acid contains by-products and impurities, such asacetic acid, formaldehyde, acrolein, propionic acid, maleic acid,acetone, furfural, and benzaldehyde, and is therefore then purified toremove these by-products and impurities, and the resultant purifiedacrylic acid which is obtained in this step and substantially free ofaldehydes is usually used as a raw material for water-absorbent resins(refer to such as JP-A-124546/1997).

However, according to the present inventors' experiences, even if theacrylic acid was purified in the above way, the properties ofwater-absorbent resins as obtained from the resultant purified acrylicacid were not always satisfactory.

Thus, the present inventors probed variously into its cause. As aresult, they found that, as is aforementioned, this purified acrylicacid contains impurities referred to as furfural and/or protoanemonin,and that these impurities have a bad influence on water absorptionproperties, and further that this bad influence is great particularly inthe case where the acrylic acid-based polymer is a neutralized salt.That is to say, the present inventors found that conventional acrylicacid contains a large quantity of furfural and/or protoanemonin, and is,from this viewpoint, still too “crude acrylic acid” to use it as a rawmaterial for the water-absorbent resin, and needs further purification.

Hereinafter referred to as superpurified acrylic acid is acrylic acidwhich is used as a raw material in the process according to the presentinvention for producing the water-absorbent resin, namely, acrylic acidof which the content of specific impurities has been reduced withattention directed to these specific impurities, in comparison with theaforementioned desolvated acrylic acid and further the purified acrylicacid as a purified product of this desolvated acrylic acid.

Thus, the present inventors did contrivance for a process for purifyingthe above acrylic acid in order to decrease the protoanemonin content,the furfural content, and the total of both contents of the acrylicacid, and probed deeply into relations of the protoanemonin and/orfurfural content of the acrylic acid with the water absorptionproperties of the resulting acrylic acid-based polymer. As a result,they found that in the case where the acrylic acid-based polymer is aneutralized salt, the decrease of the protoanemonin content andfavorably the furfural content and more favorably the total of bothcontents of acrylic acid (which is a raw material for the above acrylicacid-based polymer) into the range of 10 ppm provides better resultswith regard to the water absorption properties. That is to say, thepresent inventors found that the satisfactory results are provided bygetting “superpurified acrylic acid” which has been purified more thanconventional purified acrylic acid.

However, in the present invention, as a matter of course, if acrylicacid and/or its salt already have had the protoanemonin content(favorably also the furfural content, more favorably also the total ofboth contents) in the range of 10 ppm in the stage just after theirproduction, this acrylic acid and/or its salt may be used, intactlywithout purification, for producing the water-absorbent resin.

The protoanemonin content (favorably also the furfural content, morefavorably also the total of both contents) of acrylic acid enough toenhance the properties and performance of the water-absorbent resin isin the range of 10 ppm, favorably 0.01 to 5 ppm, more favorably 0.05 to2 ppm, particularly favorably 0.1 to 1 ppm.

Incidentally, in the case where the below-mentioned strong-alkalitreatment is carried out, the protoanemonin or furfural content isavailable even if it is more than 10 ppm and further even more than 20ppm, but more decrease thereof is favorable.

Examples of purification methods to obtain the “superpurified acrylicacid”, which has been purified more than the aforementioned “purifiedacrylic acid” which is conventionally used as raw acrylic acid, includedistillation and crystallization of which the purifiability has beenfurther enhanced. The rectification degree is, for example, enhanced by:in the case of the use of the distillation as the purification method,increasing the amount of hydrazine compound (e.g. hydrazine hydrate,phenylhydrazine, hydrazine sulfate, hydrazine hydrochloride) as added asan aldehyde-treating agent, or increasing the number of theoreticalplates (for example, by 6 to 20 plates), or increasing the reflux ratio;and, in the case of the use of the crystallization as the purificationmethod, increasing the number of times of the crystallization (forexample, by 3 to 10 times). The distillation and the crystallizationmight be used in combination with each other. The superpurificationwhich is carried out in the above way provides enablement for reductionof the protoanemonin content (favorably also the furfural content, morefavorably also the total of both contents) of purified acrylic acid froma conventional level into the range of 10 ppm and for the use of theresultant purified acrylic acid for obtaining an excellentwater-absorbent resin.

Separately from the above, it is also possible to reduce theprotoanemonin content of acrylic acid into the range of 10 ppm by addingsuch as nitrogen oxides or nitrosobenzene to the desolvated acrylic acidand thereby removing protoanemonin (refer to JP-B-041614/1981).

In the first process according to the present invention, it is favorableto further carry out an alkali treatment after intentionally reducingthe protoanemonin or furfural content in the above way. In other words,in the first process according to the present invention, it is favorableto use acrylic acid of which the protoanemonin content, more favorably,the furfural content as well, have been reduced by the alkali treatment,particularly, strong-alkali treatment. Its favorable modes are, forexample, the following (1) to (3):

(1) A specific impurity is decreased, and then the strong-alkalitreatment is carried out, and then the polymerization is carried out.

If the content of the specific impurity in acrylic acid is not more than20 ppm, this acrylic acid is available for the production of thewater-absorbent resin after being subjected to the above specificstrong-alkali treatment. The content of the specific impurity in acrylicacid is reduced to favorably not more than 10 ppm, more favorably 5 ppm,still more favorably not more than 3 ppm, yet still more favorably notmore than 2 ppm, particularly favorably not more than 1 ppm.

(2) A specific impurity is decreased, and then the alkali treatment iscarried out, and then the polymerization is carried out.

In the case where the alkali treatment is carried out with a weak baseor in a neutralization ratio of less than 100%, it is necessary to moredecrease the impurity. The content of the specific impurity is reducedto not more than 10 ppm, more favorably not more than 5 ppm, still morefavorably not more than 3 ppm, yet still more favorably not more than 2ppm, particularly favorably not more than 1 ppm.

(3) A specific impurity is decreased, and then the polymerization iscarried out without alkali treatment.

This is a process in which acrylic acid that is either substantiallyunneutralized or neutralized in an extremely small quantity (of not morethan 30 mol %) is polymerized, and this process is commonly called “acidpolymerization followed by neutralization. It has been found out by thepresent inventors that, in the case of such acid polymerization, thespecific impurity needs to more strictly be controlled. The specificimpurity is reduced to favorably 5 ppm, more favorably not more than 3ppm, still more favorably not more than 2 ppm, particularly favorablynot more than 1 ppm.

In the present invention, the strong-alkali treatment of acrylic acid isalso favorably usable for a treatment of acrylic acid which contains analdehyde such as furfural or benzaldehyde, particularly, furfural. Thatis to say, it has been known so far that the aldehyde which is animpurity of acrylic acid causes polymerization hindrance. However, ithas been found out by the present inventors that the strong-alkalitreatment according to the present invention greatly enhances thepolymerizability of the aldehyde-containing acrylic acid as such givinga water-absorbent resin.

That is to say, in the second process according to the presentinvention, the above strong-alkali treatment enhances thepolymerizability of acrylic acid as such giving a water-absorbent resin,and further enhances the properties and coloring, wherein the content ofthe aldehyde, particularly, furfural, in the acrylic acid is not lessthan 1 ppm, more favorably not less than 10 ppm, still more favorably inthe range of 20 to 500 ppm, and, at its height, 100 to 400 ppm.

In the second process according to the present invention, it isfavorable to subjecting the above aldehyde-containing acrylic acid tothe alkali treatment, particularly, strong-alkali treatment with such assodium hydroxide, favorably at the above temperature and for the abovetime, in a neutralization ratio of more than 100 mol %. It has beenfound out by the present inventors that the acrylic acid, which has beentreated for the below-mentioned time and at the below-mentionedtemperature in a neutralization ratio of more than 100 mol %, exhibitsthe enhanced polymerizability, and further, provides better results withregard to the properties. Specifically, the strong-alkali treatmentaccording to the present invention is carried out in a neutralizationratio of favorably not less than 100 mol %, more favorably not less than105 mol %, when the temperature is favorably not lower than 20° C., morefavorably not lower than 30° C., particularly favorably not lower than40° C., and when the time is favorably not shorter than 30 minutes, morefavorably not shorter than 1 hour, still more favorably not shorter than5 hours.

The alkali treatment, as referred to in the present invention, means atreatment in which acrylic acid to be treated is added to a large amountof alkali or combined with an alkali at once, and in which acrylic acidthat contains the aforementioned impurities is treated at not lower thana certain temperature under alkaline conditions, particularly understrong alkaline conditions. The polymerization of acrylic acid isgreatly promoted by such an alkali treatment. In addition, the abovealkali treatment is applied essentially to the second process, but issimilarly favorable for enhancements of the polymerizability of acrylicacid and the properties of the resulting water-absorbent resin in thefirst process as well.

Hereinafter, protoanemonin and/or furfural are referred to as specificimpurities. In the alkali treatment, not less than 50 mol %, favorablynot less than 60 mol %, more favorably not less than 65 mol %,particularly favorably not less than 70 mol %, of acrylic acid whichwill be polymerized is subjected to the alkali treatment.

In the first and second processes according to the present invention,examples of basic substances as used for the alkali treatment include(hydrogen)carbonate salts, alkaline-metal hydroxide portions, ammonia,and organic amines. However, for more enhancing the polymerizability andobtaining a water-absorbent resin having still higher properties, thestrong-alkali treatment specifically with the alkaline-metal hydroxidessuch as sodium hydroxide, potassium hydroxide, and lithium hydroxide isfavorable, and sodium hydroxide is particularly favorable. These alkalitreatments are favorably carried out in a state where the neutralizationratio of acrylic acid is more than 100 mol %, in other words, in analkali excess state, and examples thereof include: a process in whichacrylic acid is gradually added to a certain amount of alkalinesubstance to go through a strong alkaline region; and a process in whichthe alkali treatment is carried out simultaneously with theneutralization by line-mixing acrylic acid and a strong alkali together.In addition, or otherwise, in a process in which an alkali is graduallyadded to a large excess of acrylic acid, acrylic acid always exists morethan the alkali, therefore the neutralization ratio is less than 100%,and there is a possibility that the polymerizability might tend to beinsufficient, and that weak bases (e.g. (hydrogen)carbonate salts) whichare often used in general for neutralization of acrylic acid might alsobe insufficient to enhance the polymerizability following the alkalitreatment.

In the alkali treatment, particularly, strong-alkali treatment, it isfavorable that acrylic acid is treated so that an aqueous solution ordispersion having a concentration of 10 to 80 weight %, more favorably20 to 60 weight %, still more favorably 30 to 50 weight %, will beformed. The time of such an alkali treatment, particularly that forachieving the neutralization ratio of more than 100 mol %, is fitlydetermined in the range of favorably 1 second to 48 hours, morefavorably 1 minute to 36 hours, still more favorably 5 minutes to 24hours, particularly favorably 30 minutes to 24 hours. In addition, theabove alkali treatment may be carried out jointly with an alkalitreatment in which acrylic acid is stored or aged in a high alkalineregion, favorably in a neutralization ratio of not less than 100 mol %,more favorably not less than 105 mol %, for a long time, in particular,favorably not shorter than 10 hours, more favorably not shorter than 20hours, particularly favorably not shorter than 40 hours.

The temperature of acrylic acid or its aqueous solution in the alkalitreatment is kept favorably not lower than 20° C., more favorably notlower than 30° C., still more favorably not lower than 40° C.,particularly favorably not lower than 50° C. As to the alkali treatment,in cases where the temperature is low and where no strong alkali is usedand further where no neutralization is carried out, the polymerizabilityis so low that inferior results are also provided with regard to theproperties even if the aforementioned superpurified acrylic acid isused. Furthermore, the alkali treatment is carried out in the presenceof oxygen for stability, favorably, in a state where the aqueous acrylicacid (or salt) solution contains oxygen in the range of 0.5 to 20 ppm,more favorably 1 to 15 ppm, still more favorably 1.5 to 10 ppm. In thecase where the oxygen content is low, there are problems of thestability of the monomer in the alkali treatment. The alkali treatmentis carried out under an oxygen or air atmosphere, favorably, whileoxygen or air is blown in or dragged in. Incidentally, the oxygencontent is measurable with a dissolved oxygen meter.

In the present invention, the water-absorbent resin may be awater-absorbent resin obtained by a process in which acrylic acid and/orits salt is used as a monomer either intactly or after being esterified,but the former is preferable to the latter.

Examples of the acrylic acid-based polymer include: poly(acrylic acids)and neutralized poly(acrylic acids) (in which part (about 25 to about 95mol %) or all of carboxyl groups are in salt form), which are obtainedby polymerizing acrylic acid or its salts; and further, copolymers asobtained by copolymerizing the acrylic acid or its salts with, forexample, water-soluble or hydrophobic unsaturated monomers such asmethacrylic acid, maleic acid, maleic anhydride, fumaric acid, crotonicacid, itaconic acid, vinylsulfonic acid,2-(meth)acrylamido-2-methylpropanesulfonic acid,(meth)acryloxyalkanesulfonic acid, and their alkaline metal salts andammonium salts, and further, N-vinyl-2-pyrrolidone, N-vinylacetamide,(meth)acrylamide, N-isopropyl(meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycol(meth)acrylate, isobutylene, and lauryl (meth)acrylate. In the presentinvention, when monomers other than acrylic acid are used, their amountis favorably not larger than 30 mol %, more favorably not larger than 10mol %, of the total with acrylic acid and its salt which are used as themajor components.

In the case where the aforementioned acrylic acid is used, a partiallyneutralized acrylic acid-based polymer is favorable as the acrylicacid-based polymer. In view of the properties, the partially neutralizedacrylic acid-based polymer is favorably a polymer of which 50 to 95 mol%, more favorably 60 to 90 mol %, of the acrylic acid moieties areneutralized. Examples of the salt include alkaline metal salts, ammoniumsalts, and amine salts. However, metal salts, particularly, alkalinemetal salts such as lithium salts, sodium salts, and potassium salts,are favorable. Of the alkaline metal salts, sodium salts and lithiumsalts are favorable. The neutralization may be carried out either tomonomers before polymerization, or to the polymer during or afterpolymerization. Furthermore, the neutralization of the monomer and thatof the polymer may be combined with each other. However, it is favorableto subject acrylic acid to the aforementioned alkali treatment.

In the process for obtaining the acrylic acid-based polymer, bulkpolymerization or precipitation polymerization can be carried out.However, carried out favorably for the performance or the easiness ofthe polymerization control is aqueous solution polymerization orreversed-phase suspension polymerization in which the above monomercomponent is used in the form of its aqueous solution. Incidentally, thereversed-phase suspension polymerization is a polymerization method inwhich liquid drops of the aqueous monomer solution are dispersed into aninert hydrophobic solvent, and the aqueous solution polymerization is apolymerization method in which the aqueous monomer solution ispolymerized directly without being dispersed into the inert solvent.Furthermore, these polymerization methods are carried out favorablyunder an atmosphere of an inert gas such as nitrogen or argon. Inaddition, the monomer component is used for polymerization afterdissolved oxygen therein has sufficiently been displaced with the inertgas. The present invention is favorably applied to the aqueous solutionpolymerization which is of high productivity and gives high propertiesbut has been found by the present inventors to greatly be influenced byprotoanemonin, and the present invention is particularly favorable forthe aqueous solution polymerization which conventionally involvesdifficulty in controlling the polymerization.

When the monomer component is used in the form of its aqueous solutionin the above reversed-phase suspension polymerization or aqueoussolution polymerization, the concentration of the monomer component inthis aqueous solution (hereinafter referred to as “aqueous monomersolution”) is not especially limited, but is in the range of favorably10 to 70 weight %, more favorably 15 to 65 weight %, still morefavorably 30 to 45 weight %, in view of the resulting properties. Inaddition, when the above aqueous solution polymerization orreversed-phase suspension polymerization is carried out, a solvent otherthan water may be used jointly therewith if necessary, and the kind ofthis solvent as jointly used is not especially limited.

When the above aqueous monomer solution is polymerized, at least one ofthe following polymerization initiators, for example, can be used: suchas potassium persulfate, ammonium persulfate, sodium persulfate, t-butylhydroperoxide, hydrogen peroxide, 2,2′-azobis(2-amidinopropane)dihydrochloride, 2-hydroxy-1-phenylpropan-1-one, and benzoin methylether.

Furthermore, a redox initiator is also available by using the abovepolymerization initiator jointly with a reducing agent which promotesdecomposition of the above polymerization initiator and thus combiningboth with each other. Examples of the above reducing agent include:(bi)sulfurous acid (or its salts) such as sodium sulfite and sodiumhydrogensulfite; L-ascorbic acid (or its salts); reducible metals (ortheir salts) such as ferrous salts; and amines. However, the reducingagent is not especially limited. That is to say, the process accordingto the present invention exhibits great effects on aqueous solutionpolymerization involving the use of radical polymerization initiatorsand/or ultraviolet polymerization initiators, and exhibits greatereffects when the polymerization initiation temperature is in the rangeof 0 to 70° C., favorably 10 to 60° C., more favorably 20 to 50° C., sothe process according to the present invention is favorably used forsuch a polymerization method.

The amount of the above polymerization initiator or reducing agent asused is usually in the range of 0.001 to 2 mol %, favorably 0.01 to 0.5mol %, of the monomer component. In the case where the amount of thepolymerization initiator or reducing agent is smaller than 0.001 mol %,there are disadvantages in that a large amount of the monomer componentremains unreacted, so the residual monomer content of the resultantacrylic acid-based polymer increases. On the other hand, in the casewhere the amount of the polymerization initiator or reducing agent islarger than 2 mol %, there are disadvantages in that thewater-extractable content or coloring of the resultant acrylicacid-based polymer increases.

Incidentally, the polymerization reaction may be carried out either byirradiating the reaction system with active energy rays, such asradiations, electron beams, and ultraviolet rays, instead of using theabove polymerization initiator, or by the joint use of these activeenergy rays with an ultraviolet polymerization initiator or the abovepolymerization initiator. Incidentally, the reaction temperature in theabove polymerization reaction is not especially limited, but is in therange of favorably 15 to 130° C., more favorably 15 to 120° C., stillmore favorably 20 to 110° C., in terms of initiation temperature andpeak temperature. In addition, whether continuously or batchwise orwhether under static or stirred conditions the polymerization is carriedout, or the reaction time or pressure, is not especially limited,either, and may fitly be set according to factors such as the respectivekinds of the monomer component and polymerization initiator and thereaction temperature.

When the polymerization is carried out, the following materials may beadded to the reaction system in an amount of 0 to 30 weight %, favorably0 to 20 weight %: various foaming agents such as carbonate salts (orhydrogencarbonate salts), carbon dioxide, nitrogen, azo compounds, andinert organic solvents; hydrophilic polymers such as starch, cellulose,their derivatives, poly(vinyl alcohol), poly(acrylic acid) (or itssalts), and crosslinked polymers of poly(acrylic acid) (or its salts);various surfactants; and chain transfer agents such as hypophosphorousacid (or its salts).

In the case where the aqueous solution polymerization is carried out,the resultant gel polymer is disintegrated into fine pieces, ifnecessary, and then favorably dried, and, if necessary, then pulverizedor classified and further then granulated.

The extent of the above drying is as follows: the solid content of theresin as determined from its weight loss on the drying (by heating 1 gof powder at 180° C. for 3 hours) is usually not less than 80 weight %and, favorably for decomposing the aforementioned specific impurities byheating or in view of the properties of the resulting water-absorbentresin, not less than 85 weight %, more favorably not less than 90 weight%, particularly favorably not less than 93 weight %, further favorablyin the range of 94 to 99 weight %. The drying temperature is notespecially limited, but may be set, for example, in the range of 100 to300° C., favorably 150 to 250° C. In addition, the drying time is notespecially limited, but leaving the polymer alone for a long time mightbring about its coloring, so the polymer is dried to the above solidcontent in as short a time as possible, favorably within 5 hours, morefavorably within 3 hours, particularly favorably within 1 hour, afterthe polymerization. Examples of usable drying methods include variousmethods such as: heat-drying; hot-air drying; vacuum drying; infrareddrying; microwave drying; drum drier drying; dehydration by azeotropywith hydrophobic organic solvents; and high-moisture drying byhigh-temperature steaming. The drying method is therefore not especiallylimited, but the hot-air drying and the high-moisture drying are morefavorable.

The shape of the water-absorbent resin as obtained in the presentinvention is not especially limited, but examples thereof include:powdery shapes such as irregular pulverized shapes and spherical shapes;and gel shapes, sheet shapes, bar shapes, fibrous shapes, and filmyshapes. In addition, the resin may be combined with or carried onmaterials such as fibrous materials.

In the case where the water-absorbent resin is powdery, theweight-average particle diameter thereof is usually in the range of 10to 2,000 μm and, favorably in view of the properties, in the range of100 to 1,000 μm, more favorably 200 to 600 μm. And further, the contentof fine particles not larger than 150 μm is favorably as low aspossible, more favorably not higher than 10 weight %, still morefavorably not higher than 5 weight %. Substantially the upper limit ofthe particle diameter is not larger than 850 μm, and the content ofparticles with particle diameters of 300 to 850 μm is favorably notlower than 50 weight %, more favorably not lower than 70 weight %,particularly favorably not lower than 80 weight %.

Incidentally, the water-absorbent resin as referred to in the presentinvention is a hydrophilic resin which has been rendered water-swellableand water-insoluble by introducing a crosslinked structure into thepolymer, and is typically obtained by polymerizing and crosslinking themonomer component and, without load, absorbs 10 to 3,000 times as largea quantity of water as its own weight in ion-exchanged water and 5 to200 times, favorably 20 to 100 times, as large a quantity of water asits own weight in a physiological saline solution, thereby forming awater-insoluble hydrogel. In addition, examples of the abovewater-absorbent resin include those which have a water-extractablecontent of not more than 25 weight %, favorably not more than 15 weight%, more favorably not more than 10 weight %, and are thereforesubstantially water-insoluble. The methods for measuring theseproperties are herein specified in the description of the examples ofsome preferred embodiments according to the present invention.

The crosslinking method as used is not especially limited, but examplesthereof include: (A) a method which involves the steps of obtaining ahydrophilic polymer by polymerization of acrylic acid and/or its saltor, if necessary, copolymerization thereof with the above water-solubleor hydrophobic unsaturated monomers, and then adding a crosslinkingagent to the resultant hydrophilic polymer during or after thepolymerization, thereby post-crosslinking the hydrophilic polymer; (B) amethod which involves radical crosslinking with radical polymerizationinitiators; and (C) a method which involves radiation crosslinking byutilizing such as electron beams. However, a favorable one is (D) amethod which involves the steps of beforehand adding a predeterminedamount of internal-crosslinking agent to acrylic acid and/or its salt orto the above water-soluble or hydrophobic unsaturated monomers ascomonomers, and then carrying out polymerization simultaneously with orafter which a crosslinking reaction is carried out. As a matter ofcourse, the crosslinking method (D) may be employed jointly with thecrosslinking methods (A) to (C).

Examples of the internal-crosslinking agent, as used in the above method(D), include N,N′-methylenebisacrylamide, (poly)ethylene glycoldi(meth)acrylate, (poly)propylene glycol di(meth)acrylate,(polyoxyethylene) trimethylolpropane tri(meth)acrylate,trimethylolpropane di(meth)acrylate, polyethylene glycoldi(β-acryloyloxypropionate), trimethylolpropanetri(β-acryloyloxypropionate), poly(meth)allyloxyalkanes, polyethyleneglycol diglycidyl ether, ethylene glycol, polyethylene glycol, andglycerol. These internal-crosslinking agents may be used either alonerespectively or in combinations with each other. Incidentally, when atleast one internal-crosslinking agent is used, it is favorable inconsideration of, for example, the absorption properties of theresultant water-absorbent resin that a compound with at least twopolymerizable unsaturated groups is used essentially during thepolymerization.

The amount of the above internal-crosslinking agent as used is favorablyin the range of 0.005 to 2 mol %, more favorably 0.01 to 1 mol %, stillmore favorably 0.05 to 0.2 mol %, of the monomer component. In the casewhere the amount of the internal-crosslinking agent is smaller than0.005 mol % or larger than 2 mol %, the desired absorption propertiesmight not be obtained.

The water-absorbent resin obtained by the process according to thepresent invention may be a surface-crosslinked one, which isparticularly favorable as a water-absorbent resin. The water-absorbentresin according to the present invention has so low a water-extractablecontent and so high an absorption capacity that excellentsurface-crosslinking effects are made on this water-absorbent resin,which therefore can display still higher properties and performance.

Herein, the “surface-crosslinking” means further forming a portionhaving high crosslinking density in a surface layer of the resin besidesa uniformly crosslinked structure inside the resin, and is carried outwith the below-mentioned surface-crosslinking agent. Thesurface-crosslinking may be done by coating or impregnating the surfaceof the resin with the surface-crosslinking agent. The water absorptioncapacity of the resin under a load is enhanced by surface-crosslinkingthe resin. The water-absorbent resin according to the present inventionhas a water absorption capacity of not less than 20 g/g for aphysiological saline solution under a load (50 g/cm², corresponding toabout 4.90 kPa). After the surface-crosslinking of the water-absorbentresin, its water absorption capacity under the load is usually not lessthan 20 g/g, favorably not less than 23 g/g, more favorably not lessthan 25 g/g. In addition, the process according to the present inventionprovides enablement for easy and stable production of a water-absorbentresin having high properties such that the absorption capacity for aphysiological saline solution under a load (20 g/cm², corresponding toabout 1.96 kPa) is also usually not less than 20 g/g, favorably not lessthan 25 g/g, more favorably not less than 28 g/g, particularly favorablynot less than 32 g/g, and that the water absorption capacity withoutload is also not less than 25 g/g, more favorably not less than 28 g/g,particularly favorably not less than 32 g/g. In addition, the liquidpermeation quantity under a load is not less than 100 g/g, favorably notless than 300 g/g, more favorably not less than 500 g/g. Incidentally,the methods for measuring these properties are herein specified in thedescription of the examples of some preferred embodiments according tothe present invention.

Various crosslinking agents are usable for carrying out the abovesurface-crosslinking. However, from the viewpoint of the properties andcoloring, examples thereof as generally used include: polyhydric alcoholcompounds; epoxy compounds; polyamine compounds; condensation productsfrom haloepoxy compounds and the polyamine compounds; oxazolinecompounds; mono-, di-, or polyoxazolidinone compounds; polyvalent metalsalts; and alkylene carbonate compounds. Of these crosslinking agents,at least the polyhydric alcohols are used favorably for maximizing theeffects of the present invention, and polyhydric alcohols having 2 to 10carbon atoms, favorably 3 to 8 carbon atoms, are used. Examples of thecrosslinking agent include: polyhydric alcohol compounds such asethylene glycol, diethylene glycol, propylene glycol, triethyleneglycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol,polypropylene glycol, 1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol,glycerol, diglycerol, polyglycerol, 2-butene-1,4-diol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol,1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine,polyoxypropylene, oxyethylene-oxypropylene block copolymers,pentaerythritol, and sorbitol; polyamidoamines, their epichlorohydrinaddition products, 2-oxazolidinone, 1,3-dioxolan-2-one,4-methyl-1,3-dioxolan-2-one, ethylene glycol diglycidyl ether, andaluminum salts. The amount of the surface-crosslinking agent as useddepends upon factors such as types of the compounds as used orcombinations thereof, but is favorably in the range of 0.001 to 10weight parts, more favorably 0.01 to 5 weight parts, per 100 weightparts of the solid content of the resin.

In the present invention, water is favorably used for thesurface-crosslinking. The amount of water, as used on this occasion,depends upon the water content of the water-absorbent resin as used, butis usually in the range of 0.5 to 20 weight parts, favorably 0.5 to 10weight parts, per 100 weight parts of the water-absorbent resin. Inaddition, in the present invention, a hydrophilic organic solvent may beused besides water. The amount of the hydrophilic organic solvent is inthe range of 0 to 10 weight parts, favorably 0 to 5 weight parts, morefavorably 0 to 3 weight parts, relative to the water-absorbent resin.The temperature of the crosslinking agent solution is set in the rangeof 0° C. to boiling point, favorably 5 to 50° C., more favorably 10 to30° C., from the viewpoint of the mixability and stability. In addition,before mixing, the temperature of the water-absorbent resin powder isfavorably in the range of 0 to 80° C., more favorably 40 to 70° C., fromthe viewpoint of the mixability. Furthermore, in the present invention,favorable of various mixing methods is a method comprising the steps ofpremixing the surface-crosslinking agent with water and/or thehydrophilic organic solvent, if necessary, and then spraying or dropwiseadding (preferably, spraying) the resultant aqueous solution to thewater-absorbent resin to mix them together. The size of liquid dropletsas sprayed is favorably not larger than 300 μm, more favorably notlarger than 200 μm. In addition, in the mixing step, there may coexistwater-insoluble finely-particulate powders or surfactants within therange not damaging the effects of the present invention.

A favorable mixing apparatus as used for the aforementioned mixing stepneeds to be able to generate great mixing power to ensure homogeneousmixing. Favorable examples of the mixing apparatus, usable in thepresent invention, include cylinder type mixers, double-wall cone typemixers, high-speed agitation type mixers, V-character-shaped mixers,ribbon type mixers, screw type mixers, fluidized-furnace rotary disktype mixers, air blow type (pneumatic type) mixers, twin-arm kneaders,internal mixers, pulverizing type kneaders, rotary mixers, and screwtype extruders.

The water-absorbent resin and the surface-crosslinking agent are mixedtogether in the above way (if necessary, the heating treatment is thencarried out) in order to crosslink the vicinity of the surface of theresin, thus obtaining a water-absorbent resin favorably having anabsorption capacity of not less than 20 g/g for a physiological salinesolution under a load of 50 g/cm² (about 4.90 kPa) or an absorptioncapacity of not less than 25 g/g for a physiological saline solutionunder a load of about 1.96 kPa. As to conditions for carrying out theabove heating treatment, the heating temperature is favorably in therange of 100 to 250° C., more favorably 150 to 250° C. The heating timeis favorably in the range of 1 minute to 2 hours. Favorable examples ofcombinations of the temperature and time include: 180° C., 0.1 to 1.5hours; and 200° C., 0.1 to 1 hour. The heating treatment can be carriedout by using conventional dryers or heating-furnaces. Examples of thedryers include channel type blending dryers, rotary dryers, disk dryers,fluidized-bed dryers, air blow type (pneumatic type) dryers, andinfrared dryers.

In the above surface-crosslinking, the adjustment of thesurface-crosslinking agent or the heating treatment conditions (such astemperature and time) can give an absorption capacity of not less than20 g/g under a load (=about 4.90 kPa) or an absorption capacity of notless than 25 g/g under a load (=about 1.96 kPa), and further, anabsorption capacity of favorably not less than 25 g/g without load.

It is also possible to give various functions to the water-absorbentresin (as obtained by the process according to the present invention) byfurther adding thereto materials such as deodorants, antimicrobialagents, perfumes, various inorganic or organic powders, foaming agents,pigments, dyes, hydrophilic short fibers, manure, oxidants, reducingagents, chelating agents, water, salts, various hydrophilic resins otherthan the poly(acrylic acids) composing the water-absorbent resinaccording to the present invention, and various hydrophobic resins, ifnecessary. The amount of these additives is fitly set according topurposes, but is, for example, in the range of 0.001 to 400 weight %,favorably 0.005 to 10 weight %, more favorably 0.01 to 1 weight %, ofthe water-absorbent resin.

The process according to the present invention provides enablement foreasy production of a water-absorbent resin having good absorptionproperties in excellent balance between the absorption capacity withoutload, the absorption capacity under a load, and the extractable content.The resulting water-absorbent resin is widely used for such asagricultural and horticultural water-retaining agents, industrialwater-retaining agents, humidity-absorbing agents, dehumidifying agents,and building materials, but its water-absorbing agent is particularlyfavorably used for sanitary materials such as disposable diapers andsanitary napkins. Furthermore, the water-absorbent resin according tothe present invention is so excellent with regard to the above threeproperties in good balance that the sanitary materials are generallyusable in a high concentration as a water-absorbent resin concentration(weight ratio of the water-absorbent resin to the total of thewater-absorbent resin and fibrous materials), for example, of 30 to 100weight %, favorably 40 to 100 weight %, more favorably 50 to 95 weight%.

(Effects and Advantages of the Invention):

The novel process according to the present invention for producing awater-absorbent resin gives a water-absorbent resin of which theresidual monomer content and the water-extractable content are both low.Therefore, this water-absorbent resin displays excellent waterabsorbency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is more specifically illustrated bythe following examples of some preferred embodiments in comparison withcomparative examples not according to the invention. However, thepresent invention is not limited thereto.

(Production of Acrylic Acid and Acrylate Salt):

PRODUCTION EXAMPLE 1

A reaction gas, as obtained by gas phase oxidation of propylene, wascollected into water in an absorption column to obtain an aqueousacrylic acid solution. Next, this aqueous acrylic acid solution wassupplied into a solvent-separating column to distil off water and lowboiling point impurities such as acetic acid with an azeotropic solvent,thus obtaining crude acrylic acid which had a protoanemonin content of250 ppm and a furfural content of 260 ppm.

This crude acrylic acid was supplied into the column bottom of a highboiling point impurities-separating column having fifty dual-flowperforated plates, and then distilled in a reflux ratio of 1 to removeprotoanemonin together with high boiling point impurities such as maleicacid and a dimer of acrylic acid (acrylic acid dimer). As a result,furfural-containing acrylic acid having a protoanemonin content of 1 ppmand a furfural content of 240 ppm was obtained from the column top.

Next, to this acrylic acid, hydrazine hydrate was added as analdehyde-treating agent in a ratio of 400 ppm to distil the resultantmixture with a simple distiller, thus obtaining superpurified acrylicacid (1) of which the furfural content had been diminished to 1 ppm(protoanemonin content=1 ppm).

Then, this superpurified acrylic acid was subjected to an alkalitreatment in the below-mentioned way, thereby obtaining a raw acrylatesalt in the form of an aqueous solution.

A distillation flask as equipped with a stirrer was charged with 2,744 gof ion-exchanged water under an air atmosphere. Then, while thetemperature of the neutralization reaction system in the flask wasmaintained in the range of 20 to 40° C. under stirred conditions, 1,390g of superpurified acrylic acid (1) and 1,480 g of 48 weight % aqueoussodium hydroxide solution were dropwise added into this flask in adropwise addition ratio of sodium hydroxide/acrylic acid=0.9 to 0.95over a period of 100 minutes at the same time as each other. After thecompletion of this dropwise addition, 160 g of 48 weight % aqueoussodium hydroxide solution was further supplied into the flask to adjustthe neutralization ratio of the neutralization reaction system in theflask to 102 mol %. Then, the temperature of the neutralization reactionsystem was adjusted to 40° C. to age the reaction mixture for 30minutes. After the completion of this aging, 499 g of acrylic acid wassupplied to the neutralization reaction system over a period of 10minutes, thus obtaining an aqueous sodium acrylate solution having aneutralization ratio of 75 mol % and a concentration of 37 weight %(protoanemonin content=1 ppm, furfural content=1 ppm, both in the rawacrylic acid). Incidentally, the dissolved oxygen content was 8 ppm.

Incidentally, the protoanemonin content and the furfural content wereanalyzed by gas chromatography.

PRODUCTION EXAMPLE 2

Superpurified acrylic acid (2) of which the furfural content had beendiminished to 1 ppm (protoanemonin content=3 ppm) was obtained in thesame way as of Production Example 1 except that the number of thedual-flow perforated plates and the reflux ratio in the high boilingpoint impurities-separating column were changed to 40 and 1.5respectively.

PRODUCTION EXAMPLE 3

Superpurified acrylic acid (3) of which the furfural content had beendiminished to 1 ppm (protoanemonin content=9 ppm) was obtained in thesame way as of Production Example 1 except that the number of thedual-flow perforated plates and the reflux ratio in the high boilingpoint impurities-separating column were changed to 40 and 1respectively.

COMPARATIVE PRODUCTION EXAMPLE 1

Purified acrylic acid (4) of which the furfural content had beendiminished to 1 ppm (protoanemonin content=30 ppm) was obtained in thesame way as of Production Example 1 except that the number of thedual-flow perforated plates and the reflux ratio in the high boilingpoint impurities-separating column were changed to 20 and 2respectively.

COMPARATIVE PRODUCTION EXAMPLE 2

Purified acrylic acid (5) of which the furfural content had beendiminished to 1 ppm (protoanemonin content=13 ppm) was obtained in thesame way as of Production Example 1 except that the number of thedual-flow perforated plates in the high boiling pointimpurities-separating column was changed to 30.

COMPARATIVE PRODUCTION EXAMPLE 3

Purified acrylic acid (6) of which the furfural content had beendiminished to 1 ppm (protoanemonin content=17 ppm) was obtained in thesame way as of Production Example 1 except that the number of thedual-flow perforated plates and the reflux ratio in the high boilingpoint impurities-separating column were changed to 30 and 0.9respectively.

COMPARATIVE PRODUCTION EXAMPLE 4

Purification was attempted by carrying out simple distillation in thesame way as of Production Example 1 except that no hydrazine hydrate wasadded to the crude acrylic acid (protoanemonin content=1 ppm, furfuralcontent=240 ppm) as obtained from the column top of the high boilingpoint impurities-separating column. However, the resultant purifiedacrylic acid (7) had a furfural content of 230 ppm and a protoanemonincontent of 1 ppm.

(Production of Water-Absorbent Resins):

Water-absorbent resins were produced in the below-mentioned way usingthe above-obtained various raw acrylate salts (aqueous sodium acrylatesolutions).

EXAMPLE 1

First, trimethylolpropane triacrylate was dissolved as aninternal-crosslinking agent in a ratio of 0.05 mol % (to the monomer)into 7,700 g of the aqueous sodium acrylate solution (protoanemonincontent=1 ppm, furfural content=1 ppm, both in the raw acrylic acid) asobtained in the way of Production Example 1. As a result, an aqueouswater-soluble unsaturated monomer solution (aqueous monomer solution(1)) (concentration=37%, neutralization ratio=75 mol %) was obtained.

This aqueous water-soluble unsaturated monomer solution was deaeratedwith nitrogen gas for 30 minutes (so that the dissolved oxygen contentwould be reduced to less than 0.5 ppm) and then supplied into a reactoras prepared by lidding a jacketed stainless-steel-made twin-arm kneaderof 10 liters in capacity having two sigma type blades. While the aqueouswater-soluble unsaturated monomer solution was maintained at 25° C., theinternal air of the reaction system was displaced with nitrogen. Next,while the reactor was heated by passing warm water of 25° C. through thejacket, potassium persulfate of the ratio of 0.12 mol % (to 1 mol of themonomer) and L-ascorbic acid of the ratio of 0.005 g/mol (to 1 mol ofthe monomer) were added as a polymerization initiator into the reactor,and then the resultant mixture was stirred.

As a result, after 5 minutes from the addition of the polymerizationinitiator, polymerization started, and after 23 minutes, the reactionsystem reached its peak temperature, when the resultant hydrogel polymerwas finely divided into diameters of not larger than 5 mm. Thereafter,the agitation was further continued to age the polymer for 20 minutes,and then the crosslinked hydrogel polymer (1) was got out. Next, theresultant finely particulate hydrogel polymer was spread onto a metalgauze of 50 meshes (mesh opening size=300 μm) and then dried at 150° C.with hot air for 3 hours. Then, the resultant dried product waspulverized with a vibration mill and further then classified intoportions passing through a JIS standard sieve of 850 μm, thus obtaininga water-absorbent resin powder (1) having an average particle diameterof about 420 μm, in which the content of particles with particlediameters of not larger than 150 μm was 3 weight %.

EXAMPLE 2

First, 1,418 g of the superpurified acrylic acid (2) (protoanemonincontent=3 ppm, furfural content=1 ppm) as obtained in Production Example2 was dissolved into 1,944 g of ion-exchanged water. Next, while theresultant solution was stirred in the range of 20 to 40° C. in a cooledflask, thereto 1,640 g of 48 weight % aqueous sodium hydroxide solutionwas dropwise added under an air atmosphere over a period of 2 hours,thus obtaining a 37 weight % aqueous sodium acrylate solution(neutralization ratio=100%, dissolved oxygen content=6 ppm). Next, 1,611g of this aqueous sodium acrylate solution (protoanemonin content=3 ppm,furfural content=1 ppm, both in the raw acrylic acid) and 184 g of thesuperpurified acrylic acid (2) (protoanemonin content=3 ppm, furfuralcontent=1 ppm), as obtained in Production Example 2, were sufficientlymixed with polyethylene glycol diacrylate (average molecular weight=478)of the ratio of 0.10 mol % (to the monomers) as a crosslinking agent and186 g of water, thus obtaining an aqueous monomer solution (2).

The resultant aqueous solution was injected into a stainless vat(length=320 mm, width=220 mm, and height=50 mm) of which the internalsurface had been coated with Teflon. In this stage, the resultantthickness of the injected aqueous solution was 25 mm. Then, dissolvedoxygen was removed from this solution to less than 0.5 ppm byintroducing nitrogen gas into the stainless vat while the temperature ofthe aqueous solution was adjusted to 25° C. by sealing the opening ofthe vat with a polyethylene film having a nitrogen-introducing inlet, anexhaust outlet, and a polymerization initiator-adding inlet, and thenimmersing the vat into a water bath of 25° C. Thereafter, nitrogen gaswas introduced into the vat at an inflow rate of 6 L/min (108 cm/min)along a longitudinal direction of the vat, and exhaust gases werecontinuously discharged from the opposite side. Then, potassiumpersulfate of the ratio of 0.13 g/(mol of monomer) and L-ascorbic acidof the ratio of 0.001 g/(mol of monomer) were injected as apolymerization initiator into the vat, of which the contents were thensufficiently mixed together by stirring them with a magnetic stirrer. Asa result, after 1 minute from the addition of the polymerizationinitiator, polymerization started, and after 8 minutes, thepolymerization peak temperature of 96° C. was observed. Then, after thispolymerization peak, aging was carried out for 20 minutes.

The resultant crosslinked hydrogel polymer (2) was pulverized with ameat chopper as fitted with dies of the aperture of 9.5 mm, and thendried at 170° C. for 60 minutes to obtain coarse particles. Thereafterthey were further pulverized and then classified with JIS standardsieves of from 850 μm (upper limit) to 150 μm (lower limit), thusobtaining a water-absorbent resin powder (2) having an average particlediameter of about 500 μm.

COMPARATIVE EXAMPLE 1

A comparative water-absorbent resin powder (1) was obtained by carryingout an alkali treatment of acrylic acid in the same way as of ProductionExample 1 except to use the acrylic acid (protoanemonin content=30 ppm,furfural content=1 ppm) as obtained in Comparative Production Example 1,and then carrying out the same procedure as of Example 1.

COMPARATIVE EXAMPLE 2

A comparative water-absorbent resin powder (2) was obtained in the sameway as of Example 2 except to use the acrylic acid (protoanemonincontent=13 ppm, furfural content=1 ppm) as obtained in ComparativeProduction Example 2.

Shown in Table 1 are the results of the measurement of the waterabsorption capacity without load, the water-extractable content, and theresidual monomer content for each of the water-absorbent resin powdersas obtained in the above way.

From Table 1, the following would be understood. The water-absorbentresin powder (1) of Example 1, as obtained using the superpurifiedacrylic acid as a raw material of which the protoanemonin content hadbeen decreased to not more than 10 ppm, has the same water absorptioncapacity, but much lower residual monomer content and much lowerwater-extractable content, when compared with the comparativewater-absorbent resin powder (1) of Comparative Example 1 as obtainedusing the conventional acrylic acid (protoanemonin content=30 ppm,furfural content=1 ppm). The comparative water-absorbent resin powder(2) of Comparative Example 2, of which the raw acrylic acid had aprotoanemonin content of 13 ppm and a furfural content of 1 ppm, is muchinferior with regard to the residual monomer content and thewater-extractable content when compared with the water-absorbent resinpowder (2) (protoanemonin content=3 ppm, furfural content=1 ppm) asobtained under the same polymerization conditions.

EXAMPLE 3

A strong-alkali treatment was carried out in the temperature range of 20to 25° C. for 1 hour by dropwise adding 180 g of the superpurifiedacrylic acid (1) (protoanemonin content=1 ppm, furfural content=1 ppm)(as obtained in Production Example 1) in sequence over a period of 1hour to an aqueous alkali solution comprising 208.33 g of 48 weight %aqueous sodium hydroxide solution and 246.8 g of ion-exchanged water. Asa result, an aqueous sodium acrylate solution (3) having aneutralization ratio of 100 mol % and a concentration of 37 weight %(dissolved oxygen content=6 ppm) was obtained.

An aqueous monomer solution (3) having a concentration of 35 weight %and a neutralization ratio of 75 mol % (alkali treatment ratio=75 mol %)was obtained by dissolving together 228.77 g of the aqueous sodiumacrylate solution (as obtained by the above alkali treatment), 21.62 gof the superpurified acrylic acid (1), 51.06 g of ion-exchanged water,and polyethylene glycol diacrylate (average n=8) as a crosslinking agentin a ratio of 0.05 mol %. Next, while being kept at 20° C., this aqueousmonomer solution (3) was deaerated with nitrogen gas till the dissolvedoxygen content was reduced to less than 0.5 ppm. Thereafter, the aqueousmonomer solution (3) was placed into a cylindrical polypropylenecontainer having a capacity of about 500 ml wherein the polymerizationcontainer would be capped and thermally insulated in an adiabatic stateunder a nitrogen atmosphere. Then, a polymerization initiator,comprising a combination of an aqueous solution of sodium persulfate ofthe ratio of 0.12 g/mol (to the monomers) with an aqueous solution ofL-ascorbic acid of the ratio of 0.008 g/mol (to the monomers), was addedto the aqueous monomer solution (3).

As a result, 1 minute later than the addition of the above initiator,polymerization started (induction time=1 minute), and the temperaturerose due to the heat of polymerization, and then 18.5 minutes later thanthe addition of the above initiator, the peak temperature temperature(100° C.) was observed. After this attainment to the peak temperature,aging was carried out for 10 minutes. The resultant crosslinked hydrogelpolymer (3) was cut into pieces of the size of several millimeters, andthen dried by heating with hot air of 170° C. for 30 minutes. Then, inthe same way as of Example 1, the resultant dried product was pulverizedand then classified, thus obtaining a water-absorbent resin powder (3).The results are shown in Table 1.

EXAMPLE 4

The alkali treatment and the preparation of an aqueous monomer solutionwere carried out in the same way as of Example 3 except that thesuperpurified acrylic acid (1) was replaced with the superpurifiedacrylic acid (2) (protoanemonin content=3 ppm, furfural content=1 ppm)as obtained in Production Example 2. Specifically, an aqueous monomersolution (4) having a concentration of 35 weight % and a neutralizationratio of 75 mol % was obtained by dissolving together an aqueous sodiumacrylate solution (4) (as obtained by the above alkali treatment of thesuperpurified acrylic acid (2)), ion-exchanged water, the superpurifiedacrylic acid (2), and the crosslinking agent in the same way as ofExample 3.

Thereafter, the aqueous monomer solution (4) was polymerized in the sameway as of Example 3. As a result, 1.5 minutes later than the addition ofthe above initiator, polymerization started (induction time=1.5minutes). The resultant crosslinked hydrogel polymer (4) was cut, dried,pulverized, and classified in the same way as of Example 3, thusobtaining a water-absorbent resin powder (4). The results are shown inTable 1.

EXAMPLE 5

The alkali treatment and the preparation of an aqueous monomer solutionwere carried out in the same way as of Example 3 except that thesuperpurified acrylic acid (1) was replaced with the superpurifiedacrylic acid (3) (protoanemonin content=9 ppm, furfural content=1 ppm)as obtained in Production Example 3. Specifically, an aqueous monomersolution (5) having a concentration of 35 weight % and a neutralizationratio of 75 mol % was obtained by dissolving together an aqueous sodiumacrylate solution (5) (as obtained by the above alkali treatment of thesuperpurified acrylic acid (3)), ion-exchanged water, the superpurifiedacrylic acid (3), and the crosslinking agent in the same way as ofExample 3.

Thereafter, the aqueous monomer solution (5) was polymerized in the sameway as of Example 3. As a result, 1.5 minutes later than the addition ofthe above initiator, polymerization started (induction time=1.5minutes). The resultant crosslinked hydrogel polymer (5) was cut, dried,pulverized, and classified in the same way as of Example 3, thusobtaining a water-absorbent resin powder (5). The results are shown inTable 1.

COMPARATIVE EXAMPLE 3

The alkali treatment and the preparation of an aqueous monomer solutionwere carried out in the same way as of Example 3 except that thesuperpurified acrylic acid (1) was replaced with the purified acrylicacid (6) (protoanemonin content=17 ppm, furfural content=1 ppm) asobtained in Comparative Production Example 3. Specifically, acomparative aqueous monomer solution (3) having a concentration of 35weight % and a neutralization ratio of 75 mol % was obtained bydissolving together a comparative aqueous sodium acrylate solution (3)(as obtained by the above alkali treatment of the purified acrylic acid(6)), ion-exchanged water, the purified acrylic acid (6), and thecrosslinking agent in the same way as of Example 4.

Thereafter, the aqueous monomer solution (3) was polymerized in the sameway as of Example 3. As a result, 1.5 minutes later than the addition ofthe above initiator, polymerization started (induction time=1.5minutes). The resultant comparative crosslinked hydrogel polymer (3) wascut, dried, pulverized, and classified in the same way as of Example 3,thus obtaining a comparative water-absorbent resin powder (3). Theresults are shown in Table 1.

COMPARATIVE EXAMPLE 4

An alkali treatment of the purified acrylic acid (6) (protoanemonincontent=17 ppm, furfural content=1 ppm) was carried out in the same wayas of Example 5 except that the sodium hydroxide was replaced withsodium carbonate. A comparative aqueous monomer solution (4) having aconcentration of 35 weight % and a neutralization ratio of 75 mol % wasobtained by dissolving together a comparative aqueous sodium acrylatesolution (4) (as obtained by the above alkali treatment), thesuperpurified acrylic acid (2), and the crosslinking agent in the sameway as of Example 4. Thereafter, this comparative aqueous monomersolution was added as an aqueous polymerization initiator solution inthe same way as of Example 3, but no polymerization started.

COMPARATIVE EXAMPLE 5

A comparative aqueous monomer solution (5) having a neutralization ratioof 0 mol % and a concentration of 35 weight % and containing thecrosslinking agent in a ratio of 0.05 mol % was obtained in the same wayas of Example 5 except that the purified acrylic acid (6) (protoanemonincontent=17 ppm, furfural content=1 ppm) was used for polymerization withthe purified acrylic acid (6) being left as it was unneutralized withoutalkali treatment. Thereafter, this comparative aqueous monomer solutionwas added as an aqueous polymerization initiator solution in the sameway as of Example 3, but no polymerization started.

EXAMPLE 6

An aqueous monomer solution (6) having a neutralization ratio of 0% anda concentration of 35 weight % and containing the crosslinking agent ina ratio of 0.05 mol % was obtained in the same way as of ComparativeExample 5 except that the acrylic acid (6) (protoanemonin content=17ppm, furfural content=1 ppm) was replaced with the acrylic acid (2)(protoanemonin content=3 ppm, furfural content=1 ppm). Thereafter, thecrosslinking agent was dissolved into this aqueous monomer solution inthe same way as of Example 3, and the resultant solution was added as anaqueous polymerization initiator solution in the same way as of Example3, but, 6.5 minutes later, polymerization started, and the attainment toits peak needed 47 minutes. The resultant crosslinked hydrogel polymer(6) was cut into pieces of the size of about 1 mm, and then mixed withsodium carbonate powder so as to have a neutralization ratio of 75 mol%, and then dried, pulverized, and classified in the same way as ofExample 3, thus obtaining a water-absorbent resin sin powder (6).

TABLE 1 Water absorption capacity Water- Polymerization peak (g/g)extractable Residual Polymerization Protoanomonin Time Temperaturewithout content monomer container content (ppm) (minutes) (° C.) load(weight %) (ppm) Example 1 Kneader 1 23 78 33 5 280 Comparative Kneader30 27 80 33 7 470 Example 1 Example 2 Vat 3 8 96 34 6 120 ComparativeVat 13 12 102 34 8 160 Example 2 Example 3 Cylindrical 1 16 98 51 22 160container Example 4 Cylindrical 3 18 103 53 25 480 container Example 5Cylindrical 9 24 98 55 26 620 container Comparative Cylindrical 17 29 9356 28 760 Example 3 container Comparative Cylindrical 17 Weak-alkalitreatment/no polymerization Example 4 container Comparative Cylindrical(Unneutralized) No alkali treatment/no polymerization Example 5container 17 Example 6 Cylindrical (Unneutralized) 47 104 54 10 980container 3

As is understood from comparisons of Examples 3 to 5 with ComparativeExamples 3 to 5 under the same polymerization conditions in Table 1, asthe protoanemonin content increases from 1 ppm to 17 ppm, thepolymerization time (time as needed till the peak) gets longer and theresidual monomer content increases, and further, the water-extractablecontent greatly increases when compared with a small increase of thewater absorption capacity, so the properties are relativelydeteriorated. In addition, as is also understood from comparisons amongComparative Examples 3 to 5 and comparisons between Examples 2 and 6, itis understood that even if the same acrylic acid is used, the alkalitreatment, particularly, strong-alkali treatment, greatly enhances thepolymerizability.

Also shown in Table 1 are the times which passed until thepolymerization peak temperatures were indicated in the polymerizationprocesses to obtain the water-absorbent resin powders in the Examplesand the Comparative Examples. As is clear from the results, in the caseswhere the polymerization was carried out using the acrylate salts havingprotoancmonin contents of more than 10 ppm, the delay of thepolymerization was caused, and the rise of the polymerization peaktemperature was also observed, and the control of the polymerization wasalso difficult.

The methods for measuring the properties and performance, as shown inTable 1, are as follows:

Water Absorption Capacity without Load:

First, 0.2 g of water-absorbent resin powder was uniformly placed into anonwoven-fabric-made bag (60 mm×60 mm), and then the bag was immersedinto a 0.9 weight % aqueous sodium chloride solution (physiologicalsaline solution). Sixty minutes later, the bag was drawn up and thendrained at 250 G (a centrifugal acceleration of 250×9.8 ms⁻²) with acentrifuge for 3 minutes, and the resultant weight W1 (g) of the bag wasthen measured. On the other hand, the same procedure as the above wascarried out without the water-absorbent resin powder, and the resultantweight W0 (g) of the bag was measured. Thus, the water absorptioncapacity (g/g) without load was calculated in accordance with thefollowing equation:water absorption capacity (g/g) without load=(W1 (g)−W0 (g))/(weight (g)of water-absorbent resin powder).

Water-extractable Content and Residual Monomer Content:

First, 0.50 g of water-absorbent resin powder was dispersed into 1,000 gof deionized water and then stirred with a magnetic stirrer for 3 hours.Then, the resultant swollen gel was filtered off with a paper filter,when the resultant filtrate contained a water-soluble polymer(poly(sodium acrylate)) which had eluted from the water-absorbent resinpowder. Thus, 50 g of the resultant filtrate was placed into a beaker of100 ml. Then added to this filtrate were 1 ml of 0.1 N aqueous sodiumhydroxide solution, 10 ml of N/200 aqueous methyl glycol chitosansolution, and 4 drops of 0.1% aqueous Toluidine Blue solution to carryout colloidal titration, thereby determining the water-extractablecontent (water-soluble polymer content) (weight % relative to thewater-absorbent resin) of the water-absorbent resin powder.

Separately, the filtrate of the water-absorbent resin powder which hadbeen stirred for 2 hours in die above way was UV-analyzed by liquidchromatography to also determine the residual monomer content (residualacrylic acid content) (ppm/water-absorbent resin powder) of thewatcr-absorbent resin powder.

Next, the water-absorbent resin powders which had been treated bysurface-crosslinking in the below-mentioned way was also evaluated bytheir performance.

EXAMPLE 7

An aqueous crosslinking agent solution comprising1,4-butanediol/propylene glycol/water/isopropanol=0.32/0.50/2.73/0.45(weight % based on water-absorbent resin powder) was mixed into 500 g ofthe water-absorbent resin powder (2) as obtained in Example 2, and theresultant mixture was heated under stirred conditions in a bath of theoil temperature of 210° C. for 45 minutes, thus obtaining awater-absorbent resin powder (7).

COMPARATIVE EXAMPLE 6

A comparative water-absorbent resin powder (6) was obtained bysubjecting the comparative water-absorbent resin powder (2) ofComparative Example 2 to the same surface-crosslinking treatment as ofExample 7.

EXAMPLE 8

A mixture liquid comprising ethylene glycol diglycidyl ether/propyleneglycol/water=0.03/3/1 (weight % based on water-absorbent resin powder)was added by spraying to 500 g of the water-absorbent resin powder (1)as obtained in Example 1, and the resultant mixture was heated understirred conditions in an oil bath for 30 minutes, thus obtaining awater-absorbent resin powder (8).

COMPARATIVE EXAMPLE 7

A comparative water-absorbent resin powder (7) was obtained by the sameprocedure as of Example 8 except to substitute the water-absorbent resinpowder (1) as obtained in Comparative Example 1. The results are shownin Table 2.

EXAMPLE 9

A mixture liquid comprising ethylene glycol diglycidyl ether/propyleneglycol/water=0.03/3/1 (weight % based on water-absorbent resin powder)was added by spraying to 500 g of the water-absorbent resin powder (3)as obtained in Example 3, and the resultant mixture was heated understirred conditions in an oil bath for 35 minutes, thus obtaining awater-absorbent resin powder (9).

COMPARATIVE EXAMPLE 8

A comparative water-absorbent resin powder (8) was obtained by the sameprocedure as of Example 9 except to substitute the comparativewater-absorbent resin powder (3) as obtained in Comparative Example 3.Incidentally, the comparative water-absorbent resin powder (3)agglomerated in the mixing step and adhered to the mixing machine morethan the comparative water-absorbent resin powder (2). The results areshown in Table 2.

Shown in Table 2 are the results of the measurement of the waterabsorption capacity without load, the water absorption capacity under aload, and the liquid permeation quantity under a load for each of theabove water-absorbent resin powders.

From Table 2, the following would be understood. In the case wheresurface-crosslinking is carried out, when compared with thewater-absorbent resin of Comparative Example 6, the water-absorbentresin according to the present invention has almost the same absorptioncapacity, but a liquid permeation quantity under a load so greatlyenhanced from 465 g to 525 g as to have high water absorptionproperties. In other words, high surface-crosslinking effects are madeon the water-absorbent resin of Example 7 according to the presentinvention, for which the reason seems to be that the water-extractablecontent of the water-absorbent resin according to the present inventionis greatly decreased.

TABLE 2 Surface- Water absorption Liquid crosslinked water- Waterabsorption capacity (g/g) under permeation absorbent resin capacity(g/g) load quantity (g) powder without load (4.90 kPa/1.96 kPa) underload Example 7 Water- 26 24/26 525 absorbent resin powder (7)Comparative Comparative water- 26 24/26 465 Example 6 absorbent resin(6) Example 8 Water- 26 24/26 — absorbent resin powder (8) ComparativeComparative water- 26 23/25 — Example 7 absorbent resin powder (7)Example 9 Water- 42 25/38 — absorbent resin powder (9) ComparativeComparative water- 40 22/35 — Example 8 absorbent resin powder (8)Example 16 Water- 43 25/38 — absorbent resin powder (16)

The measurement methods for the data of Table 2 are as follows:

Water Absorption Capacity Under Load:

The water absorption capacity of the water-absorbent resin powder forphysiological saline solution under a load of 50 g/cm² (about 4.90 kPa)or 20 g/cm² (about 1.96 kPa) was measured in accordance with the methodsas disclosed in the working examples portions of EP 0885917, EP 0811636,and U.S. Pat. No. 6,207,772B, namely, in the following way.

An amount of 0.900 g of water-absorbent resin powder was allowed toabsorb a physiological saline solution over a period of 60 minutes in astate where a load of 50 g/cm² (about 4.90 kPa) or 20 g/cm² (about 1.96kPa) was uniformly applied to the water-absorbent resin powder, and thenthe weight W2 (g) of the absorbed physiological saline solution wasmeasured with a balance. From this weight W2, the water absorptioncapacity (g/g) under the load was calculated in accordance with thefollowing equation:water absorption capacity (g/g) under load=(W2 (g)/weight (g) ofwater-absorbent resin powder).

Liquid Permeation Quantity Under Load:

With a measurement apparatus of FIG. 1 used, 0.900 g of water-absorbentresin powder was put into a cell 41 and therein swollen with artificialurine (1) to measure the quantity of 0.69 weight % aqueous sodiumchloride solution 33 passing through the resultant swollen gel 44(mainly between particles thereof) over a period of 10 minutes under aload of 0.3 psi (about 2.07 kPa) as the liquid permeation quantity (g)under load.

The artificial urine (1), as used above, comprises 0.25 g of calciumchloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesiumchloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammoniumdihydrogenphosphate, 0.15 g of diammonium hydrogenphosphate, and 994.25g of pure water.

As to the above measurement apparatus, a glass tube 32 is inserted in atank 31, and the lower end of this glass tube 32 is positioned so as tobe maintained at the level of 5 cm above the bottom of the swollen gel44 in the cell 41. The liquid 33 is supplied into the cell 41 through anL 34 having a cock. Under the cell 41, a vessel 48 to collect the liquidwhich has passed through the swollen gel 44 is put on an even balance49. The inner diameter of the cell 41 is 6 cm, and a No. 400 stainlessmetal gauze 42 (mesh opening size=38 μm) is set at the bottom of thecell 41. A pressing portion of a piston 46 has openings 47 of the sizeenough for the liquid 33 to pass through the openings 47, of which thebottoms are fitted with a filter 45 which has good permeability butthrough which the water-absorbent resin powder or the swollen gelthereof cannot enter the openings 47. The cell 41 is supported on astand 43, of which the face contacting the cell 41 is a stainless metalgauze that does not hinder the permeation of the liquid.

The liquid permeation quantity (g) under load was determined by readingfrom the even balance 49 the quantity (g) of the liquid that flowed overa period of 10 minutes since opening the cock 35.

Needless to say, the larger the liquid permeation quantity (g) of thewater-absorbent resin under load is, the greater is the degree such thatthe water-absorbent resin serves as an excellent water-absorbent resinwhich displays, even in diapers, so high liquid permeability as to causelittle leakage and is therefore suitable also for diapers having highpolymer concentration.

EXAMPLE 10

The purified acrylic acid (7) (furfural content=230 ppm) was subjectedto a strong-alkali treatment. Specifically, an alkali treatment ofacrylic acid was carried out in a neutralization ratio of 100 mol % inthe kept liquid temperature range of 20 to 30° C. by dropwise adding 180g of the acrylic acid (7) (furfural content=230 ppm) to an aqueoussolution comprising 208.33 g of 48 weight % sodium hydroxide and 246.8 gof ion-exchanged water under an air atmosphere and under cooledconditions. An aqueous monomer solution (10) having a concentration of35 weight % and a neutralization ratio of 75 mol % was obtained bydissolving together 228.77 g of aqueous sodium acrylate solution (10)(dissolved oxygen content=6 ppm) (as obtained by the above alkalitreatment), 21.62 g of the superpurified acrylic acid (1), 51.06 g ofion-exchanged water, and polyethylene glycol diacrylate (average n=8) asa crosslinking agent in a ratio of 0.05 mol %.

Thereafter, the aqueous monomer solution (10) was polymerized in thesame way as of Examples 3 to 8. As a result, 1.5 minutes later than theaddition of the above initiator, polymerization started (inductiontime=0.5 minute). The resultant crosslinked hydrogel polymer (10) wascut, dried, pulverized, and classified in the same way as of Example 3,thus obtaining a water-absorbent resin powder (10). The results areshown in Table 3.

EXAMPLE 11

A strong-alkali treatment of acrylic acid was carried out in aneutralization ratio of 105 mol % in the temperature range of 20 to 30°C. in the same way as of Example 10 except that the amount of theacrylic acid as subjected to the alkali treatment was changed from 180 gto 171.36 g. Then, 8.64 g of the superpurified acrylic acid (1) ofProduction Example 1 was added to an aqueous sodium acrylate solutionhaving a neutralization ratio of 105 mol % (as obtained in the aboveway), thus obtaining an aqueous sodium acrylate solution (11) having aconcentration of 37 weight % and a neutralization ratio of 100 mol %.

Thereafter, this aqueous sodium acrylate solution (11) was used toobtain an aqueous monomer solution (11) in the same way as of Example10, and then this aqueous monomer solution (11) was polymerized in thesame way as of Example 10, and then the resultant crosslinked hydrogelpolymer (11) was cut, dried, pulverized, and classified in the same way,thus obtaining a water-absorbent resin powder (11), which had a residualmonomer content of 960 ppm. The results are shown in Table 3.

EXAMPLE 12

An alkali treatment was carried out for 16 hours in the same way as ofExample 11 except that the obtained aqueous sodium acrylate solutionhaving a neutralization ratio of 105 mol % (dissolved oxygen content=6ppm) was further aged at room temperature under stirred conditions for16 hours.

Then, 8.64 g of the superpurified acrylic acid (1) of Production Example1 was added to an aqueous sodium acrylate solution having aneutralization ratio of 105 mol % (as obtained in the above way), thusobtaining an aqueous sodium acrylate solution (12) having aneutralization ratio of 100 mol %. Thereafter, an aqueous monomersolution (12) was obtained in the same way as of Example 11, and thenpolymerization was carried out in the same way, and then the resultantcrosslinked hydrogel polymer (12) was cut, dried, pulverized, andclassified in the same way, thus obtaining a water-absorbent resinpowder (12). The results are shown in Table 3.

EXAMPLE 13

A strong-alkali treatment was carried out in the same way as of Example11 except that the temperature range of the strong-alkali treatment,namely, the liquid temperature range of 20 to 30° C., was changed to theliquid temperature range of 40 to 50° C. Then, 8.64 g of thesuperpurified acrylic acid (1) of Production Example 1 was added to anaqueous sodium acrylate solution having a neutralization ratio of 105mol % (as obtained in the above way), thus obtaining an aqueous sodiumacrylate solution (13) having a neutralization ratio of 100 mol %.Thereafter, an aqueous monomer solution (13) was obtained in the sameway as of Examples 11 to 12, and then polymerization was carried out inthe same way, and then the resultant crosslinked hydrogel polymer (13)was cut, dried, pulverized, and classified in the same way, thusobtaining a water-absorbent resin powder (13). The results are shown inTable 3.

EXAMPLE 14

A strong-alkali treatment was carried out in the same way as of Example13 except that, after the change of the temperature range of thestrong-alkali treatment, namely, the liquid temperature range of 20 to30° C., to the liquid temperature range of 40 to 50° C., aging wasfurther carried out at room temperature under stirred conditions for 16hours. Then, 8.64 g of the superpurified acrylic acid (1) of ProductionExample 1 was added to an aqueous sodium acrylate solution having aneutralization ratio of 105 mol % (as obtained in the above way), thusobtaining an aqueous sodium acrylate solution (14) having aneutralization ratio of 100 mol %. Thereafter, an aqueous monomersolution (14) was obtained in the same way as of Examples 11 to 13, andthen polymerization was carried out in the same way, and then theresultant crosslinked hydrogel polymer (14) was cut, dried, pulverized,and classified in the same way, thus obtaining a water-absorbent resinpowder (14), which had a residual monomer content of 740 ppm. Theresults are shown in Table 3.

EXAMPLE 15

An alkali treatment was carried out in the same way as of Example 12except that the aging at room temperature under stirred conditions for16 hours was replaced with aging at 50° C. for 16 hours. Then, 8.64 g ofthe superpurified acrylic acid (1) of Production Example 1 was added toan aqueous sodium acrylate solution having a neutralization ratio of 105mol % (as obtained by the above alkali treatment at 50° C. for 16hours), thus obtaining an aqueous sodium acrylate solution (15) having aneutralization ratio of 100 mol %. Thereafter, an aqueous monomersolution (15) was obtained in the same way as of Examples 11 to 14, andthen polymerization was carried out in the same way, and then theresultant crosslinked hydrogel polymer (15) was cut, dried, pulverized,and classified in the same way, thus obtaining a water-absorbent resinpowder (15), which had a residual monomer content of 460 ppm. Theresults are shown in Table 3.

COMPARATIVE EXAMPLE 9

Polymerization was carried out in the same way as of Example 10 exceptthat what was polymerized was a comparative aqueous monomer solution (9)having a neutralization ratio of 0 mol % and a concentration of 29weight % (corresponding to the same molar concentration as that inExample 10) which was obtained from the acrylic acid (furfuralcontent=230 ppm) without alkali treatment. However, it took not lessthan 5 minutes to initiate the polymerization, and further it took notless than 3 hours to attain the peak of the polymerization.

COMPARATIVE EXAMPLE 10

An alkali treatment of the acrylic acid (furfural content=230 ppm) wascarried out in the same way as of Example 10 except that the 48 weight %sodium hydroxide was replaced with sodium carbonate. The resultantcomparative aqueous monomer solution (10) having a neutralization ratioof 75 mol % and a concentration of 35 weight % was polymerized in thesame way as of Example 10. However, it took not less than 5 minutes toinitiate the polymerization, and further it took not less than 3 hoursto attain the peak of the polymerization.

EXAMPLE 16

A mixture liquid comprising ethylene glycol diglycidyl ether/propyleneglycol/water=0.1/3/1 (weight % based on water-absorbent resin powder)was added by spraying to 500 g of the water-absorbent resin powder (14)as obtained in Example 14, and the resultant mixture was heated understirred conditions in an oil bath for 30 minutes, thus obtaining awater-absorbent resin powder (16). The results are shown in Table 2.

TABLE 3 Polymerization peak time Strong-alkali (minutes)/ Waterabsorption Water-extractable treatment temperature capacity (g/g)content time (minutes) (° C.) without load (weight %) Color of resinExample 10 100 mol % * 20–30° C. 145/83 48 12 Yellow Example 11 105 mol% * 20–30° C. 140/70 44 12 Yellow Example 12 105 mol % * 20–30° C. + rt. 75/87 52 18 Yellow 16 hrs Example 13 105 mol % * 40–50° C.  40/95 55 23Light yellow Example 14 105 mol % * 40–50° C. + rt.  37/99 65 23 Lightyellow 16 hrs Example 15 105 mol % * 40–50° C. + 50° C. * 16 hrs  22/10457 24 Light yellow Comparative None (It took not less than 5 minutes toinitiate the polymerization.) Example 9 Comparative Weak-alkali (It tooknot less than 5 minutes to initiate the polymerization.) Example 10treatment

As is understood from Table 3, in the case where the acrylic acid havinga furfural content of 230 ppm was not subjected to alkali treatment, theprogress of the polymerization is so slow as not to be fit for theproduction (Comparative Examples 9 & 10).

On the other hand, in Examples 10 to 15 in which strong-alkalitreatments were carried out, the polymerization runs in about 1 minute(it took 145 minutes to attain the peak in Example 10). Furthermore, ifthe temperature or neutralization ratio in the strong-alkali treatmentis raised to prolong the time, then the polymerization time is greatlyshortened (to 37 minutes in Example 14/to 22 minutes in Example 15), andthe residual monomer content is also reduced, and, in Example 15, thepolymerizability is exhibited to almost the same degree as acrylic acidhaving an aldehyde content of less than 10 ppm. Furthermore, as it goeson from Examples 10 to 15 in which the alkali treatment was more andmore strengthened in this order, the water absorption capacity is alsomore and more enhanced, and coloring of the resultant water-absorbentresins is also more and more improved, and the properties resultant fromthe surface-crosslinking treatment are also more and more excellent asis shown in Table 2.

Various details of the invention may be changed without departing fromits spirit not its scope. Furthermore, the foregoing description of thepreferred embodiments according to the present invention is provided forthe purpose of illustration only, and not for the purpose of limitingthe invention as defined by the appended claims and their equivalents.

1. A process for producing a water-absorbent resin by polymerizing amonomer solution including acrylic acid and/or its salt as majorcomponents, where the acrylic acid is obtained by catalytic gas-phaseoxidation of propylene and/or propane, wherein the water-absorbent resinis a neutralized product salt having a neutralization ratio of not lessthan 50 mol % and has a crosslinked structure, wherein the acrylic acidis neutralized and polymerized by at least one of steps (1), (2) or (3),(1) acrylic acid containing protoanemonin of not more than 20 ppm issubjected to a strong-alkali treatment using an alkali-metal hydroxideto obtain an aqueous solution having a neutralization ratio of not lessthan 50 mol %, and then the polymerization of the solution is carriedout, (2) acrylic acid containing protoanemonin of not more than 10 ppmis subjected to an alkali treatment to obtain an aqueous solution havinga neutralization ratio of not less than 50 mol %, and then thepolymerization of the solution is carried out, wherein the alkalitreatment is carried out by using a weak base or carried out bygradually adding strong base to an excess of acrylic acid under thecondition of a neutralization ratio of less than 100 mol %, (3) thepolymerization of the solution of acrylic acid containing protoanemoninof not more than 5 ppm and having a neutralization ratio of 0 to 30 mol% is carried out, and then the obtained polymer is neutralized to obtaina neutralized product salt having a neutralization ratio of not lessthan 50 mol %.
 2. A process according to claim 1, wherein the acrylicacid has a furfural content of not more than 10 ppm.
 3. A processaccording to claim 1, wherein the acrylic acid is neutralized andpolymerized by the step (1) or (2).
 4. A process according to claim 1,wherein the alkali-treatment of the step (2) is a strong-alkalitreatment using an alkali-metal hydroxide.
 5. A process according toclaim 1, wherein the polymerization is aqueous solution polymerization.6. A process according to claim 1, which further comprises the step ofcrosslinking the vicinity of the surface of the water-absorbent resin.7. A process according to claim 1, wherein the resultant water-absorbentresin has a water absorption capacity of not less than 25 g/g under aload of about 1.96 kPa.
 8. A process according to claim 1, wherein themixture of said acrylic acid and/or its salt and said alkali of the step(1) or (2) is a solution containing 0.5 to 20 ppm of oxygen.
 9. Aprocess according to claim 1, wherein acrylic acid is treated in thealkali treatment of the step (1) or (2) so that an aqueous solution ordispersion having a concentration of 10 to 80 weight % will be formed.10. A process according to claim 1, wherein said water-absorbent resinhas a weight-average particle diameter in the range of 200 μm to 600 μm,and has a content of fine particles not larger than 150 μm or not higherthan 5 weight %.
 11. A process according to claim 1, wherein saidwater-absorbent resin has a water extractable content of not more than15 weight %.
 12. A process according to claim 1, wherein saidstrong-alkali treatment of step (1) is carried out at a temperature ofnot lower than 40° C.
 13. A process according to claim 1, wherein thestrong-alkali treatment of the step (1) or the alkali treatment of thestep (2) is carried out by the process where acrylic acid is graduallyadded to a certain amount of alkaline substance to go through a strongalkaline region, or the alkali treatment is carried out simultaneouslywith the neutralization by line-mixing acrylic acid and a strong alkalitogether.
 14. A process according to claim 1, wherein the polymerizationis carried out after the step of displacing dissolved oxygen from thesolution with an inert gas.
 15. A process according to claim 1, whereinthe water-absorbent resin has a liquid permeation quantity of not lessthan 300 g/g under a load of about 2.07 kPa.